Osmolyte accumulation regulates the SUMOylation and inclusion dynamics of the prionogenic Cyc8-Tup1 transcription corepressor

Environmental stressors can severely perturb cellular homeostasis and compromise viability. To cope with environmental stressors, eukaryotes have developed distinct signaling programs that allow for adaptation during different stress conditions. These programs often require a host of post-translational modifications that alter proteins to elicit appropriate cellular responses. One crucial protein modifier during stress is the small ubiquitin-like modifier SUMO. In many cases, however, the functions of stress dependent protein SUMOylation remain unclear. Previously, we showed that the conserved Saccharomyces cerevisiae Cyc8-Tup1 transcriptional corepressor complex undergoes transient hyperosmotic stress-induced SUMOylation and inclusion formation, which are important for appropriate regulation of hyperosmotic-stress genes. Here, we show the osmostress-responsive MAP kinase Hog1 regulates Cyc8 SUMOylation and inclusion formation via its role in the transcriptional activation of glycerol biosynthesis genes. Mutations that ablate Cyc8 SUMOylation can partially rescue the osmosensitivity of hog1Δ cells, and this is facilitated by inappropriate derepression of glycerol-biosynthesis genes. Furthermore, cells specifically unable to synthesize the osmolyte glycerol cause transient Cyc8 SUMOylation and inclusions to persist, indicating a regulatory role for glycerol to reestablish the basal state of Cyc8 following adaptation to hyperosmotic stress. These observations unveil a novel intersection between phosphorylation and SUMOylation networks, which are critical for shifting gene expression and metabolic programs during stress adaptation.

环境应激原可严重扰动细胞稳态并损害细胞活力。为应对环境应激，真核生物已演化出多种特异性信号转导程序，使其能够在不同应激条件下实现适应性调控。此类程序通常依赖一系列翻译后修饰，通过改变蛋白质特性以触发恰当的细胞应答反应。应激过程中一类关键的蛋白质修饰因子为小泛素样修饰蛋白（small ubiquitin-like modifier, SUMO）。然而在多数情况下，依赖于应激状态的蛋白质SUMO化修饰的具体功能仍未明确。此前本团队的研究证实，保守型酿酒酵母（Saccharomyces cerevisiae）Cyc8-Tup1转录共抑制复合物会发生短暂的高渗应激诱导的SUMO化修饰及包涵体形成，该过程对于高渗应激相关基因的精准调控具有重要意义。本研究进一步发现，渗透压响应性丝裂原活化蛋白激酶（MAP kinase）Hog1可通过调控甘油生物合成基因的转录激活，进而介导Cyc8的SUMO化修饰与包涵体形成。消除Cyc8 SUMO化修饰的突变可部分挽救hog1基因缺失（hog1Δ）细胞的渗透压敏感表型，而这一效应依赖于甘油生物合成基因的异常去阻遏。此外，无法合成渗透剂甘油的细胞会出现Cyc8 SUMO化修饰及包涵体持续存在的现象，表明甘油在细胞适应高渗应激后可发挥负调控作用，使Cyc8恢复至基础状态。上述研究结果揭示了磷酸化与SUMO化修饰网络间的全新交叉调控通路，该通路对于应激适应过程中重塑基因表达与代谢程序至关重要。